1. Field of the Invention
This invention relates to a non-contact integrated circuit (hereinafter "IC") card system which transmits and receives signals between an interrogator (a reader-writer) and a transponder (a card), in particular, to a transmitter-receiver used for the interrogator.
2. Description of the Related Art
Non-contact IC card systems are well known which transmit and receive signals between an interrogator (a reader-writer) and a transponder (a card) by the electromagnetic coupling method or electromagnetic induction method.
FIG. 3 is a block diagram commonly used in such an electromagnetic coupling type non-contact IC card system. In the non-contact IC card system shown in FIG. 3, the interrogator modulates a carrier wave generated in an OSC (oscillator) 1, through a modulating circuit 3 in accordance with signals from a data-processing circuit 2, and the modulated wave is amplified in a constant-current driver circuit 4 and then transmitted to the transponder through constant-current drive of an antenna coil 5. The transponder receives signals from the interrogator through an antenna coil 11, whereupon the received signals are demodulated in a demodulating circuit 13 and are sent to a data-processing circuit 14. The data are processed in the data-processing circuit 14, and the preset data are sent to a modulating circuit 15 and then sent out to the interrogator through the antenna coil 11. The interrogator receives the signals sent from the transponder through the antenna coil 5, whereupon the signals are demodulated in a demodulating circuit 6 and then sent to the data-processing circuit 2 to process the data to make judgement or the like.
Now, in order to enhance the selectivity of signals sent from the interrogator, the transponder is provided with a tuned resonant circuit A comprised of the antenna coil 11 and a capacitor 12. The resonant frequency f.sub.0 of this resonant circuit A depends on inductance L of the antenna coil 11 and capacitance C, and is determined by the formula: ##EQU1## where f.sub.0 is in hertz, L is in henrys, and C in farads. Thus, in order to obtain a desired resonant frequency, the antenna coil 11 and the capacitor 12 are required to have the desired inductance and capacitance.
However, the precision of capacitance of the capacitor 12 is about plus-minus 5% at best. Also, as the form of the antenna coil 11, it can be roughly grouped into two types, one of which is of the type of a wound wire and the other of which is of the type of an etched copper-clad plate. Of these, the precision of inductance of the wound wire type is about plus-minus 5%. Hence, there is a limit on the improvement in the precision of the resonant frequency f.sub.0 and an error of about plus-minus 5% usually occurs between the carrier wave frequency of the interrogator and the resonant frequency of the transponder.
Meanwhile, in order to efficiently convert into voltage the magnetic field induced in the antenna coil 11, it is more preferable for the resonant circuit to have a higher Q (quality factor). However, making the Q of the resonant circuit A higher results in a narrow frequency bandwidth selected in the resonant circuit A. Hence, if the Q is made too high, the frequency selected in the resonant circuit A turns outside the range of the tolerance between the carrier wave frequency of the interrogator and the resonant frequency of the transponder, thereby causing the problems that the signals from the interrogator can not be received.
Accordingly, the Q of the resonant circuit A is intentionally lowered to the extent that the error between the carrier wave frequency of the interrogator and the resonant frequency of the transponder does not come into question.
Alternatively, in order to set the resonant frequency of the transponder to have the desired frequency, a special adjustment is made when the transponder is manufactured. That is, a plurality of capacitors are provisionally provided as the capacitor 12 of the resonant circuit A, and some unnecessary capacitors are removed at the time of the adjustment so that the desired resonant frequency F.sub.0 can be obtained, or a troublesome operation is made, e.g., the antenna coil 11 is unwound at the time of the adjustment.